Rotate camera using eulers
Problem solved! Check the following code: 
(1) Step one: Select camera and change the transformation orientation into 'Local'
(2) Step two: Write in Python:

cam = bpy.data.objects["Camera"]

cam.select = True 

bpy.ops.transform.rotate(value=-1, constraint_axis=(False, False, True), constraint_orientation='LOCAL', mirror=False, proportional='DISABLED', proportional_edit_falloff='SMOOTH', proportional_size=1)

print(cam.rotation_euler)


Here you can change -1 into any angle you want to roll